DEVELOPMENT OF A WAVE ENERGY CONVERTER WITH MECHANICAL POWER TAKE-OFF: MODELING AND EXPERIMENTS

Abstract

This thesis presents an experimental investigation on the hydrodynamic performance and energy conversion efficiency of a highly efficient wave energy converter using a simple conceptual design. The system is based on a novel mechanical device power take-off (PTO) so-called a bidirectional rotary motion converter (BRMC), which can absorb wave energy by converting bidirectional motion of ocean waves into one-way rotation of an electric generator. First, a prototype system is designed, fabricated and assembled in the Research Institute of Small & Medium Shipbuilding (RIMS). The tests are carried out under different conditions, such as wave profiles, the resistive load coefficients and supplementary masses. A wave simulator is controlled to make harmonic waves with different amplitudes and frequencies. Some metal plates are added and fixed on the buoy as supplementary masses. Torque close-loop control has been applied on the Magneto-Rheological (MR) brake to simulate the induced torque of an electric generator. Moreover, the rotary angle compared to vertical direction, is adjusted to investigate the influence of surge mode and heave mode combination on the absorption energy. Next, the output power is calculated and compared with maximum absorbed power in heave mode to evaluate the efficiency of the prototype under different conditions. Finally, at some optimum conditions, the efficiency of the PTO system can reach 80.4% included frictional loss, and the capture width ratio is up to 41.6%. Secondly, the mathematical model of the proposed PTO system is then derived. A combined hydrodynamic and mechanical simulation of the WEC based on the time domain using pre-computed hydrodynamic coefficients is used to investigate the system operation. The hydrodynamic forces are calculated using precomputed hydrodynamic coefficients that were obtained by WAMIT. The friction behavior is considered to increase the accuracy of the simulation model. A comparison of the analytical model and recorded experimental data indicate reasonable agreement on the buoy elevation, induced torque, torque simulator speed, and generated energy. Finally, optimum control strategies are analyzed to improve the performance of the current WEC under different experimental cases. Changing the natural frequency of the PTO, finding the optimum external load, calculations of inertia for suitable flywheels, and machinery improvements are taken into account in this study. Consequently, the results indicated that the performance of the system was improved significantly, and the great potential of the new configuration means it can be applied in practical WEC applications.